This happens only without Minecart Experiments.

Steps to reproduce (a video is attached):

1. Build two rail lines, north-south and east-west. I would recommend using powered rails (and powering them).
2. Spawn a really large amount of minecarts (minecart cluster) on any rail.
3. Push the minecart cluster in the direction of your choosing.
4. Note that the minecart cluster bounces back when it begins travelling in the north and west directions. (If it didn't work, try spawning more minecarts.)